Drilling holes in the ground is often undertaken in searching for oil or other hydrocarbon sources. In such drilling it is usual to provide a drilling fluid (sometimes called a drilling mud) circulated down the hole being drilled e.g. within the drill string, around the drill bit and returning to the surface. The drilling fluid serves to cool and lubricate the bit and to carry drilling cuttings away from the bit. The hydrostatic pressure exerted by the drilling fluid is often also used to balance pressure within the rock formation through which the hole is being drilled, most notably when a pressurised fluid e.g. water, oil or gas, is contained within the rock. Commonly water, often as a brine, is the main fluid component of the drilling fluid. Particularly with aqueous drilling fluids, lubricant additives are commonly included in the fluid to improve the lubrication properties of the fluid, particularly to improve lubrication of or around the drill bit.
We have found that the inclusion of certain hydrocarbyl polyether phosphate esters in combination with polyalkylene glycols, can give aqueous drilling fluids which provide good lubricating properties in a wide range of drilling fluids, in particular where the drilling fluid has a brine phase which is sufficiently concentrated and/or includes materials which can cause incompatibility with conventional lubricants. Incompatibility can be relevant in commercially important drilling fluids, and can arise with concentrated brines, particularly saturated or near saturated brines or with the presence of materials such as calcium, particularly as calcium chloride; formate salts; and soluble silicate salts which can be used in very highly alkaline brines.